Hello, teachers!
I know this is among FAQs, but please help me confirm this!
Would you please tell me which is correct? In my guess, only #2 & #4 are correct. Am I wrong?

1. To collect stamps is my hobby.
2. Collecting stamps is my hobby.
3. My hobby is to collect stamps.
4. My hobby is collecting stamps.
5. It is my hobby collecting stamps.
6. It is my hobby to collect stamps.

Thank you very much.
That's right. 2 and 4 are the correct ones.
The others would be understood, but we don't say it that way.

It has to do with whether you're talking about an activity (-ing) or an event or achievement (to ...). I've never thought about it before, but it seems to me that the formulation with the infinitive is used for loftier expressions, nothing as trivial as a hobby and a stamp collection. But I may be wrong about this, and the activity/achievement distinction probably takes precedence.

To conquer the world was Alexander's ambition.
His ambition was to conquer the world.
It was his ambition to conquer the world.

Mary's goal is to comfort the sick and the elderly.
To become knowledgeable about the workings of government should be the goal of every citizen.

Chasing one another around the house was the children's favorite game.
(not "to chase one another ..."!)

It was his goal to collect every stamp ever printed in Australia. (a more ambitious project!)

Joseph's secret desire was to photograph the Queen of England. (accomplishment)
Photographing the Queen of England was Paul's job. (activity)

Emotion: smile
Hi, Jim.
Thank you for your looong, sincere, informative, helpful answer with good example sentences! But as a bonehead, I still have a question, please help me again!Emotion: smile

How about these? Which is the correct, natural one(s)?

1. My hobby is collecting stamps and playing tennis.
2. My hobbies are collecting stamps and playing tennis.
3. Collecting stamps and playing tennis are my hobbies.
4. Collecting stamps and playing tennis are my hobby.
5. It is my hobby collecting stamps and playing tennis.
6. They are my hobbies collecting stamps and playing tennis.

Thank you very much.
Enjoy the happy weekend!